This invention relates to a wire scribed circuit board product and method of manufacture. Specifically this invention relates to wire scribed circuit boards wherein at least some of the conductors are coaxial conductors.
Coaxial conductors are the preferred transmission medium for high frequency signals, particularly high speed digital signals, in which the signal conductor is protected by a shield which is electrically connected to the reference potential or "ground". A coaxial conductor contains all of the signal energy inside its shield on the signal conductor. The shield isolates the signal conductor from the surrounding environment, thus providing a clean uniform environment for signal travel. The shield prevents radiation of energy from the signal conductor which improves signal propagation and reduces electrical interference which may affect other signals in the vicinity.
Although coaxial conductors are recognized as a preferred interconnect medium where high signal switching speeds are involved, they are not widely used in electrical circuit boards because of the problems incurred in properly terminating the signal conductor at the end points of the coaxial conductor. The signal conductor and shield must be electrically isolated from each other at the terminal points. In manual installations, the signal conductor is usually terminated by stripping back or removing the shield and insulation and connecting it to the terminal point by soldering, welding, or the like. Termination of the shield is conventionally made by connecting a wire, often called a drain wire, between the shield and a central ground point. This approach, however, is not practical where there is a large number of interconnections on a circuit board.
Wire scribed circuit boards can be produced using the method described in U.S. Pat. Nos. 3,671,914 and 3,674,102. In this process an insulated wire is scribed onto an insulating base by feeding a continuous strand of wire onto the surface of the base while simultaneously affixing the wire to the base thereby producing a wire image of a predetermined interconnect pattern. Through holes are subsequently drilled at the terminal points intersecting the end points of the conductors. The holes are then metallized to electrically connect the end points of the conductors to surface terminal pads or later added components.
The above cited patents on methods for manufacture of discrete wire circuit boards broadly describe a method for adhering insulated wire to an insulating base or substrate and for making terminations at the ends of the conductors. The process, however, does not work if coaxial conductors are used as the interconnect medium since the signal conductor and shield would become electrically connected or "shorted" at the terminal points.
In U.S. Pat. No. 3,969,816 "Bonded Wire Interconnection System" by Swengel et al, discrete coaxial conductors are placed at locations on a substrate. Lengths of coaxial conductors are bridged between discrete point locations determined by a matrix apertured substrate. The conductors are anchored within the substrate apertures and the ends then milled flush with the board surface exposing both the shield and the signal conductor. In order to prevent shorting of the signal conductor to the shield at the termination, a ring of dielectric material is placed on the surface, covering the shield. In another embodiment a selective etch (apparently a photo resist with selective activation) is used to recess the shield from the surface plane. The patent also discloses terminal points interconnected by insulated conductors which are metallized to create a shield for the conductors. the conductor termination techniques disclosed in the '816 patent are all based on having conductor terminations flush with the board surface.